Ultra tough maraging steel



July 26, 1966 E. P. SADOWSKI ULTRA TOUGH MARAGING STEEL 2 Sheets-Sheet 1 Filed July 13, 1964 SE23? on Q (/g cm/j HJSAUJJQ 072 Z30 Kw. mm w W m United States Patent 3,262,777 ULTRA TOUGH MARAGIN G STEEL Edward Peter Sadowski, Riugwood, N.J., assignor to The International Nickel Company, Inc., New York, N.Y., a corporation of Delaware Filed July 13, 1964, Ser. No. 382,309 14 Claims. (Cl. 75-124) The present invention relates to ferrous-base alloys of high yield strength and, more particularly, to high strength steels which are exceedingly tough, the combination of toughness and high yield strength rendering the steels particularly suitable for use in the form of steel plate and welded structures fabricated therefrom.

As is well known to those skilled in the metallurgy of steel, the quest for steels manifesting higher levels of strength combined with good toughness has witnessed little interruption and research efforts to that end have been greatly intensified in recent yea-rs. To a considerable extent, this concerted emphasis has evolved from the need for steels characterized by high strength-to-weight ratios whereby the uneconomical dead weight factor is minimized, thus affording steels capable of competing more effectively with materials of inherently less weight. In addition, few, if any, technological developments in present day steel metallurgy evolve Without giving considerable, if not paramount, emphasis to the toughness characteristics of the steels. Of course, this aspect is not at all surprising in view of the fact that high strengths per se are readily attainable as is evident from the substantial number of steels known to possess yield strengths of 150,000 pounds per square inch (p.s.i.) or above. As a general proposition (and excluding the stainless types), such steels can be perhaps classified broadly into at least two principal categories: the rather moderate to high carbon (0.2% to about 0.45% carbon) heat treated low alloy steels (including the normalized, the quenched and tempered and the precipitation hardening types) and the mar-aging steels introduced a few years ago.

With regard to the class of heat treated carbon-containing, low alloy steels (herein referred to as the HTC steels), many vexatious problems have arisen and continue to prove to be a source of difliculty. In developing high yield strength levels of 150,000 p.s.i. and above, application of liquid quenching operations has been more than common. This focuses attention on the problems of warpage and dimensional change, problems which must be obviated by the application of costly and burdensome processing techniques, e.g., the use of jigging equipment during heat treatment. Elimination of the quenching operation is obviously desirable to avoid the problems attendant thereto but in many HTC steels it is the quenching operation whicr is usually necessary to develop the hardenability and strength characteristics conferred by carbon, carbon being the element exerting the most potent influence in such steels.

Moreover, the HTC steels are, as is well known, susceptible to weldability and embrittlement problems. With respect to the former, as the carbon content is increased, welding difficulties become more pronounced and, in this connection, it is the desideratum, commercially speaking, to keep the carbon content at a level of below about 0.3%, and preferably not much more than about 0.2%. But, as indicated above, it is the higher carbon contents which afford the high strength levels. In a sense, competing factors are involved. A further distinct disadvantage to achieving good weldability characteristics with the HTC steels arises in connection with the fabrication of large structures such as huge pressure vessels and marine hulls. Such structures are generally formed from steel plates (e.g., /2 inch to several inches in thickness) welded to each other. Due regard being given to maintaining high strength levels, the problems connected with preand postweld heat treating of such structures reaches the point of practical impossibility from a commercial viewpoint. If high strength HTC steel plates were welded to form a large pressure vessel, it would be generally necessary to postheat the weldand the zone of parent metal adjacent or contiguous thereto (often referred to as the heat-affected zone). A postheattreatment is considered not only desirable in providing sound welds but it would be generally necessary to subject the weld metal and contiguous parent metal to a quench operation to restore the high strength level expunged by the high heat of the welding operation. In other words, the quench would be necessary to effect a transformation back to martensite and avoid retained anstenite. Suflice to say, to quench a huge vessel is virtually impossible. This probably accounts, at least partially, for the fact that pressure vessels of substantial size are 7 generally formed of steels having yield strengths of less than about 100,000 p.s.i. where the problems are not as accentuated.

The aforementioned embrittlement problem is generated particularly in connection with the quenched and tempered steels. That is to say, when a steel is quenched and thereafter tempered, cooling through certain temperatures from tempering temperatures results 'in embrittlement. While a complete panacea is not now known which might obviate the embrittlement problem in all cases, it can be said that there has been a tendency, at least with regard to-many HTC steels, to use low tempering temperatures, e.g., 500 F. and lower, with a view of avoiding the embrittling temperature range. However, low tempering temperatures result in lower tensile elongation, reduction of area and the ability to absorb impact energy. These properties concern the more general charactristic of toughness.

While, as referred to above herein, yield strength levels of 150,000 p.s.i. and above can be readily attained, achieving the same together with good toughness poses a different and distinct problem since in steel metallurgy an increase in strength is usually associated with a concomitant decrease in toughness. In this connection, it is of fundamental importance that certain metallurgical terms be brought into proper perspective. For example, it is quite common to report toughness characteristics on bar, rod and the like as distinct from plate. With regard to the former, it is well known that tensile ductilities, reductions of area, and impact strengths (as determined by, for example, Charpy V-Notch standards), are significantly higher than those for plate specimens made out of the same steel. It is further noteworthy to mention that the ability of steel plate to absorb impact energy is quite different when measured in the longitudinal and transverse directions. Reference to the 1961 edition of the Metals Handbook, pages 229-231, reflects what is a generally acknowledged fact, to Wit, that Charpy V-Notch values determined in a longitudinal direction, i.e., the direction parallel to the direction of rolling, are usually substantially higher than those determined from the trans verse direction. While the gap between longitudinal and transverse results can be narrowed by cross rolling, this ope-ration is not devoid of difiiculty and cannot always be employed. In any event, it would not be without benefit for a designer to have at his disposal an indication of the ability of a steel plate to absorb impact energy in a direction transverse to that of the direction of rolling.

With respect to the maraging steels, the type of steels to which the present invention is directed, it can be said that the introduction thereof has opened up a rather vast new area of metallurgy. The exceedingly high strengths obtainable together with good ductility and other mechanical characteristics coupled with the fact that quenching of the steels is unnecessary have already led to various commercial applications. In exploiting the potential of these steels, as is the case with most new developments, research eiforts have been expanded in an effort to provide maraging steels of improved toughness characteristics.

In US. application Serial No. 286,365 of which I am a co-applicant, maraging steels of 140,000 p.s.i. to about 190,000 p.s.i. are described with impact energy absorption levels (Charpy V-Notch values in steel plate measured in the transverse direction) of up to 45 foot-pounds (ft-lbs.) being obtained at the highest yield strength. Insofar as I am aware, such steels in comparison with known prior art HTC and maraging steels afforded a combination of toughness and yield strength not obtainable prior thereto.

Notwithstanding the excellent properties of maraging steels hitherto known and as described in US. application Serial No. 286,365, it has now been discovered that by controlling the boron and/or zirconium contents thereof a very marked and substantial improvement is obtained thereover. In fact, at comparable yield strength levels, e.g., 175,000 to 190,000 p.s.i., alloys in accordance with the instant invention manifest a degree of toughness of upwards of about 35% to nearly 100% greater than that characteristic of the steels in the above-noted U.S. application. It has been also found that toughness characteristics are further improved by maintaining the silicon content of the steels at a low level, as will be shown herein, and that an optimum combination of toughness and strength is attained by the incorporation of special amounts of columbium in the steels.

It is pertinent and basic to the instant invention, as those skilled in the specific art of maraging steels are aware, it be understood that from the outset of the original development of the maraging steels, boron and zirconium have been added to such steels mainly for deoxidizing and malleabilizin-g purposes, although they also contribute (to a minor extent) to the strength thereof. These additions were made much in the sense that small amounts of silicon and manganese are added to the carbon and low alloys steels. Often little or no specific mention is made of these elements in the literature pertaining to maraging steels and this is obviously understandable since it was deemed that these elements merely performed the function of affording deoxidation characteristics and the like. It should be added that it has been rather standard practice to add about 0.003% boron and/or about 0.03% zirconium to the maraging steels. This is done regardless of the fact that the recovered amount of, for example, boron, might be expected to be higher due, for example, to pickup from furnace linings, refractories, etc., used for melting. Such amounts of these elements are indeed small; nonetheless, as will be demonstrated herein, such amounts have now been found to exert a subversive influence in achieving optimum levels of toughness. Actually, the qualitative effect which evolved from exercising special control over either or both of these constituents was not expected and, at

' zirconium, with the balance being essentially iron.

present, the complete theory which might explain this rather strange behavior of boron and zirconium is not wholly understood.

It is an object of the present invention to provide a novel maraging steel of improved toughness.

Another object of the invention is to provide via a simple heat treatment a novel maraging steel characterized by an unusual combination of toughness and strength characteristics of an extremely high order of magnitude.

It is a further object of the invention to provide novel welded plate structures fabricated from the steels contemplated herein.

Other objects and advantages will become apparent from the following description taken in conjunction with the accompanying drawings in which:

FIGURE 1 graphically depicts a general and illustrative relationship between the toughness and strength characteristics of the steels contemplated herein and the aluminum content thereof; and

FIGURE 2 is another graphic representation which is generally illustrative of the effect of boron and zirconium and also silicon on impact strength of steels of the invention.

Generally speaking, and in accordance with the present invention, the alloy steels contemplated herein consist essentially (in percent by weight) of about 9.5% to 13.5% nickel, about 2.5% to about 8% chromium, about 1.9% to 4.2% molybdenum, up to 0.75% aluminum, e.g., about 0.05% to about 0.7% aluminum, up to about 0.3% titanium, up to 0.25% columbium, carbon in an amount up to about 0.03%, e.g., 0.001% to 0.03% carbon, up to 0.25 manganese, up to 0.5% silicon, up to not more than 0.0015% boron, up to not more than 0.01% and advantageously up to not more than 0.005% As will be readily understood by those skilled in art, the term balance or balance essentially when used to indicate the amount of iron in the alloy steels does not exclude the presence of other elements commonly present as incidental elements, e.g., deoxidizing and cleansing elements, and impurities ordinarily associated therewith in small amounts which do not adversely atfect basic characteristics of the steels. In this connection, elements such as sulfur, phosphorus, hydrogen, oxygen, nitrogen and the like should be kept at low levels consistent with commercial steelmaking practice. However, auxiliary elements such as beryllium, vanadium, tantalum and tungsten can be utilized in the steel of the instant invention. Such elements when used singly should not exceed the following amounts: 0.2% beryllium, 1% vanadium, 0.8% tantalum and 1% tungsten. When two or more auxiliary elements are utilized the total sum thereof should not exceed 2%. Incidental elements such as cobalt and copper do not provide any particular attributes and, if desired, can be limited to those small amounts unavoidably introduced during commercial processing.

In achieving an optimum combination of properties, including yield strength and toughness, e.g., a minimum yield strength of about 175,000 p.s.i. together with a minimum Chanpy V-Notch impact energy level of about ft.- lbs. (room temperature) on steel plate /a inch thick and as measured in the transverse direction, it is most advantageous that the alloy steels contain about 11.5% to 12.5% nickel, about 4.5% to about 5.5% chromium,

about 2.75% to 3.25% molybdenum, about 0.3% to 0.6% aluminum, about 0.15% to about 0.25 titanium, up to about 0.2% columbium, about 0.001% to 0.03% carbon, up to not more than 0.001% boron, up to not more than 0.005 zirconium, up to 0.1% silicon, up to 0.1% manganese, with the balance being essentially iron. Within these ranges the steels upon aging exhibit yield strengths of up to about 200,000 p.s.i. or above, together with Charpy V-Notch impact values of at least 50 ft.-l-bs. at this strength level. Energy impact levels of about fL-lbS.

and up to about 100 ftfilbs. on plate specimens taken from the transverse direction can be attained at lower yield strength levels, e.g., 170,000 p.s.i.

The nickel content of the steels should not fall below about 9.5%; otherwise there is an undesirable decrease in strength. Nickel contents greater than about 13.5% do not afford an appreciable advantage. Further, high nickel contents coupled with chromium contents on the high side of the chromium range could efliect a loss in strength. In this connection, the sum of the nickel and chromium contents should be at least about 14% and not greater than 19%. Chromium in an amount of less than 2.5% also results in lower strength levels. Amounts of molybdenum appreciably above the maximum specified herein adversely affect the toughness of the steels and there is a concomitant loss in strength. At least 1.9% and advantageously at least 2.75% molybdenum should be present for a good combination of toughness and yield strength.

Whereas in U.S. patent application Serial No. 286,365 it was deemed that the aluminum content of the alloys contemplated therein could not exceed 0.4%, and preferably should not exceed 0.35%, in order to obtain a good level of toughness, it has now been found that amounts well above 0.4%, e.g., 0.45%, and up to 0.75% are distinctly advantageous in achieving high strength levels. However, aluminum is a most potent performer in the steels of the invention and where an optimum of strength and toughness is necessary, the aluminum content should not exceed 0.6%. While yield strengths well above 200,000 p.s.i. are readily attainable with aluminum contents of 0.7% to 0.75%, a rather precipitous drop in toughness is experienced particularly at 0.75% aluminum as is shown in FIG. 1. It will be noted the point of intersection of the curves in FIG. 1 shows a Charpy V-Notch energy level of about 68 ft.-lbs. together with a yield strength of about 188,000 p.s.i. This represents an increase in toughness of about 40% over and above the value given by the intersection of the curves in FIG. 1 of the aforesaid U.S. patent application where a Charpy V-Notch value of about 47 to 48 ft.-lbs. is set forth at about the same yield strength level of 188,000 p.s.i.

Apart from controlling the amounts of boron and zirconium present in the alloys, it is most preferred, as will be shown herein, that the silicon content not exceed about 0.12%, e.g., 0.1%. Silicon in amounts as low as 0.25% effects a marked reduction in toughness characteristics. While silicon contents of up to 0.35% or 0.5% can be used, it is significantly more advantageous to limit the silicon content to 0.1% Where optimum toughness is desired.

In addition to the foregoing, it has been found that small but effective amounts of columbium up to 0.25% enhance the ability of the steels to absorb higher levels of impact energy. This higher level of impact toughness is achieved without loss in other desirable properties including yield strength.

A further most satisfactory compositional range of alloying constituents contemplated herein is as follows: about 10.5% to 13% nickel, about 3.5% to 7.5% chromium, about 2.25% to 3.75% molybdenum, about 0.2% to 0.65% aluminum, up to 0.25% titanium, up to 0.2% columbium, e.g., 0.05% to 0.15% columbium, carbon up to 0.03%, up to not more than 0.0012% boron, up to 0.007% zirconium, up to 0.15% silicon, up to 0.15% manganese, and the balance essentially iron.

In carry-ing the invention into practice and while it is preferred for a number of applications to use vacuum processing techniques to assure the minimum presence of gases, a further virtue of the steels of the invention is that they can be produced utilizing common air-melting and of itself precludes, as a practical matter, the use of, for example, consumable electrode vacuum processing since this latter technique does not readily lend itself to the production on a commercial basis of large size heats, e.g., heats Weighing 75 tons and above. Use of relatively high purity alloying ingredients is beneficial. Upon melting of the basic charge, i.e., nickel, chromium, molybdenum and iron, and after completion of the carbon boil, calcium can be used to effect a desulfurization, although the use of calcium is not necessary in vacuum processing. Aluminum, titanium, silicon, manganese and, if any, boron and zirconium, are then added. Upon solidification the cast ingots should be homogenized as by soaking at temperatures of about 2100 F. to 2350 F., e.g., 2300 F., followed by hot working and, if desired, cold working to desired shape. The steels can suitably be worked at temperatures of 2000 F. to 1800 F., the finishing temperature being about 1700 F. to 1450 F., e.g., 1550 F. to 1500 F. Preferably the steels are transformed to martensite by cooling to about room temperature and then applying a solution anneal treatment. While the final hot working temperature can be used in lieu of an anneal and the steels can then be aged after cooling from hot-working temperature, it is considerably more beneficial to effect a second martensitic transformation by cooling from hot working and then annealing followed by cooling. This procedure results in the attainment of tougher steels. The solution annealing treatment comprises subjecting the steels to a temperature of about 1450 F. to 1900 F. for up to about 4 hours, e.g., /2 to 4 hours. In producing sheet material periods of less than 1 hour can be used. It is preferred to employ a solution anneal temperature between about 1500 F. to 1600 F. for about 1 to 3 hours. Upon cooling to about room temperature from the annealing temperature the steels pass through the M M range and, thus, again transform into .the martensitic condition. Thereafter, the steels are aged at temperatures of about 800 F. to 1000 F. for about 1 to 24 hours or longer. It is advantageous that the aging temperature not exceed about 950 F. to avoid reversion to austenite. Long aging times at the higher aging temperatures are not recommended less overaging and/or reversion to austenite occur. A recommended aging treatment consists of holding the steels at a temperature of about 875 F. to 925 F. for 2 to 4 hours. The steels can be cooled to below room temperature prior to aging, e.g., down to minus F., as by, for example, refrigeration. This technique can be adopted to assure complete transformation to martensite. However, this treatment is generally unnecessary in accordance with the invention, i.e., substantially complete transformation occurs upon cooling from solution treatment. Cold working prior to aging can also be used to effect the completion of transformation to martensite.

For the purpose of giving those skilled in the art a better understanding of the invention and/or a better appreciation of the advantages thereof, the following illustrative description and data are given:

Several alloy steels having compositions either within or without the invention were prepared and are identified in Table I, Alloys Nos. 1 through 5 being within the invention and Alloys A through M being outside the scope thereof. Neither boron nor zirconium was added to steels 1 through 5; however, small amounts of these elements were recovered (or picked up, as the term is used) from the furnace lining, or otherwise represent the impurity levels in the materials used. In this regard, the recovered boron did not exceed 0.0012% and the recovered zirconium Was less than 0.005%. Both boron and zirconium were added in the usual and standard deoxidizing amounts of about 0.003% and 0.03%, respectively, in Alloys A through G. Boron only (0.003%) was added to Alloys H, I and J and zirconium only (0.03%) was added to Alloys K, L and M. The respec- TABLE I Chemical analysis being given in foot-pounds (ft.-lbs.). In this connection, the Charpy V-Notch impact energy absorption test was conducted on specimens taken transverse to the one-direction rolled plate.

NO BORON OR ZIRCONIUM ADDED Alloy No. 0, Ni, Cr, Mo, Al, Ti, Si, Mn, B, Zr,

Percent Percent Percent Percent Percent Percent Percent Percent Percent Percent BORON AND ZIRCONIUM ADDED BORON ADDED ZIRCONIUM ADDED Balance of steels was iron except for impurities, e.g., sulfur (less than 0.005% in all alloys) and phosphorus (less than 000.35% in all alloys).

The alloy steels in Table I were melted in a vacuum induction furnace. Subsequent to solidification, cast ingots (4 x 4 inches) were soaked at 2300 F. to achieve good homogenization and then forged to 2 x 2 /2 inch specimens with two intermediate soaks at 2300 F. being employed. The specimens were than one-direction rolled in three passes to steel plate inch thick. The initial rolling temperature was 1800 F. with the rolling being finished at a temperature of about 1750 F. to 1650 F.

The steel plates were allowed to air coo-l to room temperature whereby the steels transformed to the martensitic condition. Each of the steels was then solutiton treated (annealed) at about 1500 F. for about 1 hour, air cooled (to again achieve the martensitic condition) and then aged at about 900 F. for about 3 hours.

After cooling from aging temperature, the steels were subjected to test and the results are reported in Table II with the yield strength (Y.S., 0.2% oflfset) and ultimate tensile strength (U.T.S.) being given in thousands of pounds per square inch (K.S.I.), the tensile elongation and reduction in area being given in percent and the Charpy V-Notch energy absorption values (C.V.N.)

TABLE II 55 Y.S., 0.2% U.'T.S., Elonga- Reduc- G. .N., Alloy N0. Offset, K.S.I. K.S.I. tion, tion of ft.-lbs.

percent Area,

percent 60 1 169.4 177.2 17. 5 70.0 92. s 2 176. 6 181. 4 17. 0 69.0 91, 5 3 184. 7 191. 3 17. 5 66. 5 72. 5 4 201. 8 205. 2 15.0 65. 5 59.2 5 180. 5 183. 9 17. 0 69. 5 80.8 A 210. 7 216. 9 16. 0 65. 5 24.0 B 208. 6 211.8 16. 0 65. 0 30. 2 5 C 203. 8 207. 4 15. 0 60. 5 32. 8 D 192. 2 196. 2 16.0 62.0 38. 0 E 190. 1 195. 3 15.0 69. 0 45. 0 F 178. 6 183. 2 16.0 62. 5 51. 2 G 173. 3 177. 6 15. 0 66. 0 56. 7 H 176. 0 180. 6 18.0 68. 5 66.8 I 182.0 186. 7 16.0 67. 5 52. 5 .I 201.8 205. 2 15.0 65. 5 40. 0 K 173. 7 176. 8 17. 0 68.0 63. 7 L 186. 6 190. 6 17. 0 66. 5 41. 2 M 199. 8 202. 4 15. 0 59.0 35. 6

While the data in Table II illustrate the surprising and boron and/or zirconium contents are controlled in accordance with the concepts of the invention, the marked improvement can be more readily ascertained by reference to Table III wherein various alloys of Table I and corresponding data of Table II are set forth in a manner which affords a more convenient comparison. The alloys given in Table III are presented on the basis that the compositions thereof are, practically speaking, essentially the same except for the boron and/ or zirconium contents (silicon and manganese are excluded from the tables so that composition plus yield strength and Charpy V-Notch impact values could be reported in one convenient table).

a magnitude believed to be comparable to the best hitherto attained. As between boron and zirconium, the latter is more subversive than the former. Accordingly, particular care should be exercised regarding zirconium control. FIGURE 2 is also included herein to graphically depict a general effect of boron and/or zirconium in the alloys of Table III.

It has previously been indicated herein that silicon even in small amounts, e.g., 0.25%, also exerts an adverse infiuence in respect of the capability of the alloys to absorb high energy impact. This effect is demonstrated by the data in Table IV, the alloys having been prepared and treated in the identical manner as the alloys given in con- TABLE III BORON AND ZIRCONIUM EFFECT Alloy No 0, Ni, Cr, Mo, Al, Ti, B, Zr, Y.S., C.V.N.,

percent percent percent percent percent percent percent percent K.S.I. it.-lbs.

BORON EFFECT ZIRCONIUM EFFECT 1 Not added.

Table III illustrates the superior ability of the alloys nection with Table 1. Alloys Nos. 1 and 5 are included (Alloys 1 through 5) of the invention to absorb impact for the purpose of comparison.

TABLE IV Alloy N o. 0, Per- Ni, Per- Cr, Per- Mo, Per- Al, Per- Ti, Per- B, Per- Zr, Per- Si, Per- Y.S C.V.N.

cent cent cent cent crnt cent cent cent cent K.S I ft.-lbs.

energy. This marked and contrasting behavior is manifested by the last column of Table III, wherein it is evievident that in every case an improvement of at least was obtained, the improvement running as high as 95%. This is deemed a significant advantage, particularly in view of the fact that high levels of yield strength werev maintained and in view of the fact that the alloys used as a basis of comparison and which contained boron and/ or zirconium in amounts outside the present invention ex- In addition to the data of Table IV, FIGURE 2 also shows the deleterious influence of silicon. However, it is to be pointed out that the impact characteristics of Alloys N and O are quite good. Whenever steels afiord yield strengths of the magnitude of 170,000 to 180,000 p.s.i.,

together with Charpy V-Notch values of about or over 50 ft.-lbs. (transverse specimens from plate), the steel, absent some other factor, must be considered excellent. Nonetheless, the most advantageous alloys herein and hibited a combination of toughness and yield strength of which contain not more than 0.15% silicon and not more 1 1 than 0.001% boron and 0.005% zirconium, respectively, must by the same token be deemed markedly superior in View of a demonstrated ability to withstand much higher levels of impact energy.

12 molybdenum, about 0.3% to about 0.6% aluminum, about 0.15% to about 0.25% titanium, up to about 0.2%, columbium, about 0.001% to 0.03% carbon, up to not more than 0.001% boron and up to not more To the foregoing should be added, as referred to above than 0.05% zirconium to thereby greatly minimize their herein, that control of carbon content is important. That influence in adversely affecting the toughness characthe carbon content should not exceed about 0.03% is teristics of said steel, up to 0.1% silicon, up to 0.1% illustrated by Table V. manganese, and the balance essentially iron.

TABLE V Alloy N o. 0, Per- Ni, Per- (Jr, Per- Mo, Per- Al, Per- Ti, Per- Si, Per- Mn, Per- B, Per- Zr, Per- Y.S., C.V.N. cent cent cent cent cent cent cent cent cent cent K.S.I. It.-lns.

5 0.022 12.08 4.73 3.15 0. as 0.23 0. 04 0.08 0. 001 0. 005 180.5 80.8 P 0. 03s 12.18 4. 60 3.15 0. 37 0.23 0.04 0.07 0.001 0. 005 177.4 66.5

As referred to herein, it is advantageous in accordance With preferred concepts of the invention that the alloy 2. The maraging steel as set forth in claim 1 wherein the aluminum is present in an amount of about 0.45%

steels contain columbium in amounts up to 0.25%. This to about 0.6%.

is demonstrated by a comparison of Alloy 5 with Alloy 6 and Alloy 6 with Alloy Q in Table VI, the alloys being prepared and treated in the same manner as the alloys of Table I.

3. A maraging steel manifesting a unique ability to absorb extremely high levels of impact energy while simultaneously affording high levels of yield strength and consisting essentially of a composition falling within TABLE VI Alloy No. 0, Per- Ni, Per- Cr, Per- Mo, Per- Al, Per- Ti, Per- Si, Per- Mn, Per- B, Per- Zr, Per- Cb, Per- Y.S., C.V.N

cent cent cent cent cent cent cent cent cent cent cent K.S.I. it.-lbs.

1 Not added.

Alloy 6 illustrates the beneficial efIect of columbium on C.V.N. energy levels in comparison with Alloy 5, an alloy of similar composition except that columbium was not added thereto. With columbium contents much above 0.25% (Alloy Q), Charpy V-Notch impact energy levels drop oil; however, Alloy 6 reflects that with amounts of columbium in accordance with the invention, impact energies of 100 ft.-lbs. can be obtained together with yield strength levels above about 180,000 p.s.i.

While the present invention is generally applicable to providing maraging steels in the forms of strip, bar, rod, sheet, etc., it is particularly applicable in providing maraging steels in plate form and in welded structures fabricated therefirom. Other structures which can be formed from the steels contemplated herein include pressure vessels, ships hulls, rotors for electrical generators, etc. The high level of toughness of steels described render the steels eminently suitable for cryogenic application down to temperatures of the order of minus 320 F. This feature is decidedly beneficial in view of the greatly expanded utilization of vessels and the like for storage of liquids and gasees at low temperature.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

I claim:

1. A maraging steel manifesting a unique ability to absorb extremely high levels of impact energy of about (a) 75 foot-pounds and above While simultaneously affording a high level of yield strength of at least about 175,000 p.s.i., and (b) about 50 foot-pounds and above while affording a yield strength of about 200,000 p.s.i., and consisting essentially of a composition falling within the following ranges: about 11.5% to 12.5% nickel, about 4.5% to 5.5% chromium, about 2.75% to 3.25%

the following ranges: about 10.5% to 13.5% nickel, about 3.5% to 7.5% chromium, about 2.25% to 3.75% molybdenum, about 0.2% to 0.65% aluminum, up to about 0.25% titanium, up to about 0.2% columbium, carbon up to 0.03%, up to not more than 0.00l'2% boron and up to not more than 0.007% zirconium to thereby greatly minimize their influence in adversely affecting the toughness characteristics of said steel, up to about 0.15% silicon, up to about 0.15 manganese, and the balance essentially iron 4. The alloy as set forth in claim 3 wherein columbium is present in an amount of 0.05% to 0.15%.

5. The alloy as set forth in claim 3 wherein the aluminum is present in an amount of about 0.45 to about 0.65

6. The alloy as set forth in claim 3 wherein the silicon content does not exceed 0.12%.

7. The alloy as set forth in claim 4 wherein the aluminum is present in an amount of about 0.45 to about 0.65%.

8. A novel maraging steel consisting essentially of about 9.5% to 13.5% nickel, about 2.5% to 8% chromium, about 1.9% to 4.2% molybdenum, up to about 0.75% aluminum, up to about 0.3% titanium, up to about 0.25% columbium, carbon in an amount up to 0.03%, up to 0.25% manganese, up to 0.5% silicon, up to not more than 0.0015 boron and up to not more than 0.01% zirconium to thereby greatly minimize their influence in adversely affecting the toughness characteristics of said steel, up to about 0.2% iberyllium, up to about 1% vanadium, up to about 0.8% tantalum, up to about 1% tungsten, with the total amount of beryllium, vanadium, tantalum and tungsten not exceeding 2%, and the balance essentially iron.

9. The steel as set forth in claim 8 wherein the sum of the nickel plus chromium is from about 14% to about 19%.

10. The steel as set forth in claim 8 and which contains aluminum in an amount of from 0.05% to 0.7%.

11. The steel as set forth in claim 8 and which con- .13 tains a small but effective amount of columbium sufficient to enhance the toughness characteristics of the steels, the columbium content not exceeding about 0.25%.

12. The steel as set forth in claim 8 in which the aluminum content is above 0.4% and up to 0.65% and in which the silicon content does not exceed 0.25

13. The steel as set forth in claim 11 in which the silicon content does not exceed 0.15% and columbium is present in an amount of about 0.05% to about 0.15

14. A novel maraging steel consisting essentially of about 9.5% to 13.5% nickel, about 2.5% to 8% chromium, about 1.9% to 4.2% molybdenum, up to about 0.75% aluminum, up to about 0.3% titanium, up to about 0.25% columbium, carbon in an amount up to 0.03%, up to 0.25% manganese, up to 0.5% silicon, at least one element selected from the group consisting of boron and zirconium, the amount of boron not exceeding 0.0015% and the amount of zirconium not exceeding 0.01% to thereby greatly minimize their influence in adversely afiecting the toughness characteristics of said steel, and the balance essentially iron.

References Cited by the Examiner DAVID L. RECK, Primary Examiner.

C. N. LOVELL, Assistant Examiner. 

1. A MARAGING STEEL MANIFESTING A UNIQUE ABILITY TO ABSORB EXTREMELY HIGH LEVELS OF IMPACT ENERGY OF ABOUT (A) 75 FOOT-POUNDS AND ABOVE WHILE SIMULTANEOUSLY AFFORDING A HIGH LEVEL OF YIELD STRENGTH OF AT LEAST ABOUT 175,000 P.S.I., AND (B) ABOUT 50 FOOT-POUNDS AND ABOVE WHILE AFFORDING A YIELD STRENGTH OF ABOUT 200,000 P.S.O., AND CONSISTING ESSENTIALLY OF A COMPOSITION FALLING WITHIN THE FOLLOWING RANGES: ABOUT 11.5% TO 12.5% NICKEL, ABOUT 4.5% TO 5.5% CHROMIUM, ABOUT 2.75% TO 3.25% MOLYBDENUM, ABOUT 0.3% TO ABOUT 0.6% TO 3.25% ABOUT 0.15% TO ABOUT 0.25% TITANIUM, UP TO ABOUT 0.2%, COLUMBIUM, ABOUT 0.001% TO 0.03% CARBON, UP TO NOT MORE THAN 0.001% BORON AND UP TO NOT MORE THAN 0.05% ZIRCONIUM TO THEREBY GREATLY MINIMIZE THEIR INFLUENCE IN ADVERSELY AFFECTING THE TOUGHNESS CHARACTERISTICS OF SAID STEEL, UP TO 0.1% SILICON, UP TO 0.1% MANGANESE, AND THE BALANCE ESSENTIALLY IRON. 